[Bugfix] Mail 047999: Fix CC/BCC Mails Generic Placeholder not replaced with corresponding value

[fhelfer]

https://mantis.ilias.de/view.php?id=47999

[mjansenDatabay]

• requiresRecipient should be part of the array shape definition (PHPDocs)
• Maybe there are other placeholders in specific mail template contexts which are also generic (like a course title). Therefore, I carefully ask: Do you see any other approach to achieve the desired behaviour?

[fhelfer]

For the interim fix, we also need to add requiresRecipient to all mail contexts. Right now there’s no reliable way to tell which specific placeholders are user-specific vs. context-specific. Generic ones already have it; specific ones don’t — and some contexts still short-circuit everything when there’s no recipient. Study Programme is the worst offender: if (is_null($recipient)) { return ''; } wipes all placeholders, even ones that only need the object/context (title, link, etc.).

Long-term (planned for ILIAS 12): a proper refactor. Placeholders become small classes implementing interfaces, collected the same way mail signatures already work — placeholder classes + a supports() check on the consumer side. Contexts would declare what they support (installation, user, object/course/study programme, …), and only matching placeholders get resolved. That way we reuse placeholders across contexts instead of reimplementing them in every il*MailTemplateContext.